Discuss The Role Played By Endogenous Pacemakers And Exogenous Zeitgebers In Biological Rhythms
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1Iain Downer Discuss the role played by endogenous pacemakers and exogenous zeitgebers in biological rhythms. A biological rhythm can is as any change in a biological activity that repeats periodically. They include rhythms with a frequency or periodicity of less than one day (ultradian), those with a periodicity of approximately one day (circadian), and those with a periodicity of greater than one day (infradian). These biological rhythms are most often synchronized with daily, monthly, or annual cyclical changes in the environment. These external factors affecting biological rhythms are known as exogenous zeitgebers. A zeitgeber is an environmental cue, such as the length of daylight or the degree of temperature that helps to regulate the cycles of any any living organism's biological clock (or endogenous pacemaker). The daily pattern of sleeping for 8 hours in every 24 (the sleep-wake cycle) is, perhaps, the most obvious of our biological cycles and is known as a circadian rhythm. A circadian rhythm is a cycle of roughly 24 hours (from the Latin circa, “around” and diem, diem, “day”) which is generated biologically, and can be affected, or synchronised depending on which zeitgebers are present, present, this is mostly light and dark. Research has suggest that in the brain, there are certain structures which act as an endogenous pacemaker for the body by detecting light and dark, thus allowing an organism to sleep and wake in synchronisation with the earth’s light-dark cycle. A number of different brain structures are involves in regulating sleeping and waking, however the main biological clock in humans and animals seems to be a small area in the hypothalamus – the suprachiasmatic nucleus (SNC) – whose neurons have an inbuilt circadian rhythmic firing pattern. The SCN regulates the production of serotonin and melatonin in the pineal gland via an interconnecting pathway. Melatonin is a hormone which induces sleep. Another pathway pathway connects the retina of each eye to the SCN. The amount of light falling on the retina influences the activity of SCN neurons and, indirectly, the release of melatonin from the pineal gland. So the link between light and melatonin production is maintained. Ralph et al. (1990) used hamsters, some with a genetic abnormality affecting their circadian cycle, to try and provide evidence that the SCN generates the circadian rhythm in mammals. A group of hamsters was identified with a genetic abnormality that was resulted in a 20-hour circadian cycle, rather than 24-hour. The SCNs were removed and placed in the brains of an experimental group of hamsters with a normal 24-hour cycle. Eventually, the experimental group shifted to a 20-hour cycle. The study supports the theory that the SCN is the endogenous pacemaker and generates the rhythm in animals. We don’t, however, have any reason to believe that the human sleep-wake cycle is radically different. A lot of studies, also involving not-human animals, were conducted in lab conditions and therefore the results cannot be generalized to an animal’s natural habitat. There are ethical issues with this study such as causing harm or distress and also, the procedure is extremely invasive. Research has shown that animals fed on a regular basis become active just befor feeding time. This happens in the absence of other environmental cues and therefore supports the existence if some sort of internal biological clock or regulator. Rossenwasser et al. (1981) found that rats still showed this anticipation after their SCN had been destroyed, so another biological clock must also be able to perform this function. However, these results have derived from studies involving animals. One important issue that must be taken into consideration generalisability to humans. We
2Iain Downer do know that systems differ from one animal to the next; therefore it is important to check any animal findings against research with humans. The most famous study of free-running biological rhythms involved a French cave explore called Michel Siffre who, in 1972, spent 6 months in an underground cave in Texas, in the absence of natural light-dark cycles. When he was awake, the researchers put the lights on; when he went to bed, they turned the lights on. He ate and slept whenever he wanted. At first his sleep-wake cycle was very erratic, but settled down to a fairly regular pattern of between 25 and 30 hours, that is, slightly longer than a 24-hour cycle. When he emerged, it was the 179th day, but by ‘his days, it was only the 151st day since he went underground. The case of Michael Siffre may be dismissed as the study of just one unusual individual. However, we should consider the important role they play in helping us understand the nature off circadian rhythms. Although unusual, such studies offer an extremely rare insight into what happens when our bodies are left to ‘free-run’, and therefore play a role in confirming what experimental studies involving larger groups have already suggested. Studies of free-running biological rhythms in humans also show that there are significant individual differences in these mechanisms, i.e. that they may not operate in exactly the same way in all people. In a similar study, Kate Aldcroft was housed in a lab for 25 days, also with no access to cues about the time of day. To indicate her perception of the passage of time, she was asked to play Amazing Grace on the bagpipes at, what she believed to be the same time, each day. The time at which she played became later over the study period. She began to sleep for longer (up to 16 hours at a time) and her sleep-wake cycle extended to 30 hours. The contrast between the alterations in the length of sleep-wake cycles is further evidence for the role of individual differences in free-running biological rhythm studies. The study of Michael Siffre might be described as a case study – it is the study of one individual and therefore has unique features. His body’s behaviour may not be typical of all people and, in addition, living in a cave living in a cave may have particular effects due to, for example, the fact that it is cold. However, subsequent studies above ground have confirmed the findings of research in cave environments (Kate Aldcroft). Aldcroft). Siffre’s study was also an experiment – he controlled key variables (exogenous zeitgebers) to observe the effects on the sleep wake-cycle. The experimental approach is important because it allows us to demonstrate causal relationships. These two studies clearly show how the body’s sleep-wake circadian rhythm is disrupted in the absence of external cues and, in effect, the role of exogenous zeitgebers in circadian rhythms. The sensitivity to light of the pineal gland and the SCN, and the role of melatonin in controlling sleep and other activity, mean that despite the endogenous nature of biological clocks, their activity is synchronized with the light-dark rhythm of the outside world. Occasionally, slightly bizarre studies, such as Michael Siffre and Kate Aldcroft, Aldcroft, have allowed us to look at the effects of removing light as an exogenous zeitgeber, allowing these biological clocks to run free. Studies such as these, show that humans with a free-running biological clock settle into a rhythmic sleep-wake pattern of between 25 -27 hours, that is, slightly longer than under normal conditions. Conclusions which can be drawn from this are that endogenous mechanisms can control sleep-wake cycles in the absence of light, but also, that light as an exogenous zeitgeber is necessary to reset the clock every day so that the biological rhythm is coordinated with the external world. There are, however, also weaknesses of this biological approach to bodily rhythms. All these studies are typical of the biological approach to understanding behaviour: behaviour:
3Iain Downer they propose that human behaviour can be explained in terms of structures in the brain and in terms of hormonal activity. However, human behaviour is often more complex than this because people can override biologically determined behaviours by making choices about what they do. In this sense, the biological approach may be seen as reductionist. On the other hand, sometimes it may not be possible to override biological factors and biological rhythms. A powerful example of this was the study of a young man who was blind from birth and had a circadian rhythm of 24.9 hours. He was exposed to various exogenous zeitgebers such as clocks and social cue, yet found it difficult to reduce his internal pace. This made it very difficult for him to function and, as a result he had to take stimulants in the morning and sedatives’ at night in order to get his biological rhythm in time with the rest of the world (Miles et al., 1997).
2Iain Downer do know that systems differ from one animal to the next; therefore it is important to check any animal findings against research with humans. The most famous study of free-running biological rhythms involved a French cave explore called Michel Siffre who, in 1972, spent 6 months in an underground cave in Texas, in the absence of natural light-dark cycles. When he was awake, the researchers put the lights on; when he went to bed, they turned the lights on. He ate and slept whenever he wanted. At first his sleep-wake cycle was very erratic, but settled down to a fairly regular pattern of between 25 and 30 hours, that is, slightly longer than a 24-hour cycle. When he emerged, it was the 179th day, but by ‘his days, it was only the 151st day since he went underground. The case of Michael Siffre may be dismissed as the study of just one unusual individual. However, we should consider the important role they play in helping us understand the nature off circadian rhythms. Although unusual, such studies offer an extremely rare insight into what happens when our bodies are left to ‘free-run’, and therefore play a role in confirming what experimental studies involving larger groups have already suggested. Studies of free-running biological rhythms in humans also show that there are significant individual differences in these mechanisms, i.e. that they may not operate in exactly the same way in all people. In a similar study, Kate Aldcroft was housed in a lab for 25 days, also with no access to cues about the time of day. To indicate her perception of the passage of time, she was asked to play Amazing Grace on the bagpipes at, what she believed to be the same time, each day. The time at which she played became later over the study period. She began to sleep for longer (up to 16 hours at a time) and her sleep-wake cycle extended to 30 hours. The contrast between the alterations in the length of sleep-wake cycles is further evidence for the role of individual differences in free-running biological rhythm studies. The study of Michael Siffre might be described as a case study – it is the study of one individual and therefore has unique features. His body’s behaviour may not be typical of all people and, in addition, living in a cave living in a cave may have particular effects due to, for example, the fact that it is cold. However, subsequent studies above ground have confirmed the findings of research in cave environments (Kate Aldcroft). Aldcroft). Siffre’s study was also an experiment – he controlled key variables (exogenous zeitgebers) to observe the effects on the sleep wake-cycle. The experimental approach is important because it allows us to demonstrate causal relationships. These two studies clearly show how the body’s sleep-wake circadian rhythm is disrupted in the absence of external cues and, in effect, the role of exogenous zeitgebers in circadian rhythms. The sensitivity to light of the pineal gland and the SCN, and the role of melatonin in controlling sleep and other activity, mean that despite the endogenous nature of biological clocks, their activity is synchronized with the light-dark rhythm of the outside world. Occasionally, slightly bizarre studies, such as Michael Siffre and Kate Aldcroft, Aldcroft, have allowed us to look at the effects of removing light as an exogenous zeitgeber, allowing these biological clocks to run free. Studies such as these, show that humans with a free-running biological clock settle into a rhythmic sleep-wake pattern of between 25 -27 hours, that is, slightly longer than under normal conditions. Conclusions which can be drawn from this are that endogenous mechanisms can control sleep-wake cycles in the absence of light, but also, that light as an exogenous zeitgeber is necessary to reset the clock every day so that the biological rhythm is coordinated with the external world. There are, however, also weaknesses of this biological approach to bodily rhythms. All these studies are typical of the biological approach to understanding behaviour: behaviour:
3Iain Downer they propose that human behaviour can be explained in terms of structures in the brain and in terms of hormonal activity. However, human behaviour is often more complex than this because people can override biologically determined behaviours by making choices about what they do. In this sense, the biological approach may be seen as reductionist. On the other hand, sometimes it may not be possible to override biological factors and biological rhythms. A powerful example of this was the study of a young man who was blind from birth and had a circadian rhythm of 24.9 hours. He was exposed to various exogenous zeitgebers such as clocks and social cue, yet found it difficult to reduce his internal pace. This made it very difficult for him to function and, as a result he had to take stimulants in the morning and sedatives’ at night in order to get his biological rhythm in time with the rest of the world (Miles et al., 1997).
